1. Field of the Invention
The present invention relates to nanoparticle films and their forming methods and applications.
2. Description of Related Art
Nanotechnology has been developed as a reliable technology for producing minimal components to perform more precise functions. In nanotechnology, nanoparticles are fundamental as building blocks to form novel materials because they have several unique features. For example, the properties of nanoparticles are determined by quantum rather than classical physics due to their small size; the surface physics of material may determine the properties of the material due to the large surface-atoms to bulk-atoms ratio; and the surface properties of the materials may be modified through self-assembled monolayer coatings.
A large variety of shape-controlled nanoparticles, including metal, semiconductor, organic, magnetic, insulating, superconductor, and so on, have been chemically synthesized in the literature; and generally they are formed or assembled on a substrate to practice its unique properties; in other words, a nanoparticle film may be defined as a film containing one, two, or three-dimensional bulk assemblies of nanoparticles.
The one, two, or three-dimensional nanoparticle assemblies of the nanoparticle films typically exhibit properties that are not present in the bulk material and that can be applied in various fields. For example, surface plasmons are coherent electron oscillations that exist at the interface between any two materials, such as a metal material and a dielectric, when the metal material is nanometer-sized, light excites the surface plasmons at the interface between the metal material and the dielectric, resulting in plasmon resonance. In the past few years, the optical measurements of various types of plasmon resonances, such as surface plasmon resonance (SPR), localized plasmon resonance (LPR), and collective plasmon resonance (CPR), have been utilized for sensing applications in chemistry and biology to detect moleculars, such as polymers, DNA, or proteins' adsorption.
Recently, considerable attention has been directed to the studies of near-field-coupled noble metal nanoparticle systems because of their tunable plasmonic properties, which are very desirable for a variety of applications. Among these systems, the coupling effects of plasmonic dimers constructed by two nanoparticles placed next to each other within the near-field range have been widely studied. For more complex systems, it has become clear that collective plasmon resonance (CPR) in coupled colloidal gold nanoparticles arrays can manifest itself as plasmonic crystal effects. In a recent work, it has been shown that the CPR modes can be generated in two-dimensional (2D) self-assembled gold nanoparticles superlattices via near-field coupling between neighboring nanoparticles in close-packed superlattices. It has also been confirmed that the CPR peak position can be sensitively tuned by varying the interparticle gap distance. Moreover, Tao et al. has demonstrated that silver nanoparticles can be used as building blocks to construct 3D plasmonic crystals. (Prodan, E., Radloff, C., Halas, N.J. & Nordlander, P. A hybridization model for the plasmon response of complex nanostructures. Science 302, 419-422 (2003); Su, K.-H., Wei, Q.-H., Zhang, X., Mock, J. J., Smith, D. R. & Schultz, S.; Interparticle coupling effects on plasmon resonances of nanogold particles. Nano Lett. 3, 1087-1090 (2003); Nordlander, P., Oubre, C., Prodan, E., Li, K. & Stockman, M. I. Plasmon hybridization in nanoparticle dimers. Nano Lett. 4, 899-903 (2004); Jain, P. K., Huang, W. & El-Sayed, M. A. On the universal scaling behavior of the distance decay of plasmon coupling in metal nanoparticle pairs: A plasmon ruler equation. Nano Lett. 7, 2080-2088 (2007); Yang, S.-C., Kobori, H., He, C.-L., Lin, M.-H., Chen, H.-Y., Li, C., Kanehara, M., Teranishi, T. & Gwo, S. Plasmon hybridization in individual gold nanocrystal dimers: Direct observation of bright and dark modes. Nano Lett., vol. 10, 632-637 (2010); Tao, A., Sinsermsuksakul, P. & Yang, P. Tunable plasmonic lattices of silver nanocrystals. Nature Nanotechnol. 2, 435-440 (2007); Chen, C. F., Tzeng, S. D., Chen, H.-Y., Lin, K.-J. & Gwo, S. Tunable plasmonic response from alkanethiolate-stabilized gold nanoparticle superlattices: Evidence of near-field coupling. J. Am. Chem. Soc. 130, 824-826 (2008); Tao, A. R., Ceperley, D. P., Sinsermsuksakul, P., Neureuther, A. R. & Yang, P. Self-organized silver nanoparticles for three-dimensional plasmonic crystals. Nano Lett. 8, 4033-4038 (2008)).
Several methods, such as drying of colloidal gold solution droplets deposited onto substrates, electrophoretic deposition, cross-linking nanoparticles with linkers, have been reported in the literature for forming nanoparticle films. For example, successful multilayered gold films made on glass supports by a layer-by-layer deposition have been reported in the literature. The conventional layer-by-layer electrostatic self-assembly is a simple yet elegant way to deposit macroscopic, multilayered nanoparticle films onto surfaces functionalized by oppositely charged (e.g., polyelectrolytes) or chemically conjugated (e.g., dithiols) cross-linkers. However, the use of cross-linkers drastically reduces the mobility of the individual nanoparticles and hinders the formation of well-ordered superlattices. Therefore, both close packing and long-range ordering are not feasible using these approaches, as evidenced in the related microscopic studies. Moreover, interlayer plasmonic coupling, which is important for the realization of 3D plasmonic metamaterials, is generally non-existent in these multilayer systems. In addition, the conventional layer-by-layer method suffers from a number of disadvantages: its procedure is too slow, other problems: amorphous structures, limited film thickness, nonuniformity.
In addition, important questions remain about the practicality of self-assembly techniques for fabricating nanoparticle films. For example, it is important to demonstrate the feasibility to deposit a single layer of highly ordered nanoparticles over a wafer-scale substrate. Until now, the largest 3D nanoparticle crystals grown by self-assembly techniques have been limited to sub-millimeter dimensions. Furthermore, it would be more controllable to deposit nanoparticle films in a novel layer-by-layer fashion, similar to the molecular-beam epitaxy technique used for fabricating semiconductor devices. If these capabilities can be realized, it would become more practical to build 3D nanoparticle films with engineerable properties, such as the plasmon resonance. Hence, one of the outstanding challenges in the art of nanotechnology is to form large-scale self-assembly of nanoparticle films which exhibit controllable collective properties. In particular, the large-scale, self-assemble, three-dimensional (3D) nanoparticle films should have strong and tunable properties, such as plasmonic response, which could allow the creation of novel materials for a variety of applications. Therefore, it would be advantageous to provide novel nanoparticle films and their forming methods and applications capable of achieving such needs.